The present invention relates generally to audio circuits, and, more specifically, to a method and device for reducing popping noises in audio circuits that drive speakers.
Since the early days of sound reproduction, audio amplifiers (amps) have been used to produce an audible output from a weak input signal. Originally, a mechanical megaphone attached to a diaphragm provided sound amplification. Then, as electrical technology advanced, vacuum tubes provided an electrical platform for amplifying an audio signal. Electrical sound amplification provides many advantages over mechanical sound applicationxe2x80x94for example, a much smaller magnetically activated speaker replaced the large mechanical megaphone. Unfortunately, electrical audio amplification introduced a distortionxe2x80x94the xe2x80x9chiss,xe2x80x9dxe2x80x9cwhistle,xe2x80x9d xe2x80x9ccrack,xe2x80x9d and xe2x80x9cpopxe2x80x9d (collectively xe2x80x9cpopping noisexe2x80x9d) which are associated with audio circuit start-up. These sounds are caused when currents in the audio circuit (or a load attached to the audio circuit) xe2x80x9cspike,xe2x80x9d meaning that the currents get too large for the speaker.
More recently, solid-state integrated circuits (ICs) have been used to reproduce and amplify audio signals. Other solid-state devices, such as digital signal processors (DSPs), have been applied to audio circuits to control the amplification of a small electrical signal into audible sound. Accordingly, ICs have enabled audio circuits to shrink in size from a unit about the size of a filing cabinet to a unit about the size and weight of a credit card. The miniaturization of audio circuits has been accomplished while simultaneously increasing the reliability of audio circuits. For example, a vacuum tube based audio circuit would often required several repairs each year, while a modern IC audio circuit may need replacement after several years of service. Furthermore, an IC audio circuit is much less expensive than previous circuits. In fact, it is much cheaper to replace an IC audio circuit than to repair any previous audio amplifier. However, IC audio circuits still have internal capacitors, which are responsible for the popping noises. Accordingly, IC based audio circuits still experience the popping noise that has been associated with turning on an audio circuit since the era of vacuum tubes.
Several schemes have been attempted in an effort to control popping noises. In one scheme, the voltage in the audio circuit is increased very slowly taking as many as thirty seconds to a full minute to increase to a full operating voltage. However, slowly increasing the voltage in the audio circuit has the obvious disadvantage of delaying the operation of the audio circuit, and will either create a long silent period for the listener, or the listener will hear a soft pop followed by a gradual increase in volume until a full volume is reached. This increase in volume is often called the open xe2x80x9cwarm-upxe2x80x9d of the audio circuit.
Another scheme limits the user""s ability to quickly increase the amplifier gain. Unfortunately, this scheme does not provide an operator of the audio circuit with the volume or power control one often needs. This is a particular problem in audio circuits that function as audio channel equalizers, such as those used in recording studios. Other proposed solutions have run into other limitations, such as limitation imposed by the number of connection pins on the audio circuit, as well as limitations due to the die space available on the audio circuit.
Therefore, what is needed is a system and method of reducing popping noise in audio circuits. It would be advantageous for the solution to use few pins on the audio circuit and would occupy a small amount of die space. The present invention provides such a system and method.
The present invention achieves technical advantages as an audio circuit and a method of operating the audio circuit that reduces popping noises during audio circuit start-up. One embodiment of the audio circuit includes a voltage generator configured as a bypass generator for producing a smooth S-shaped voltage- on the non-inverting input of an audio amplifier, which is used to monitor a reference voltage. A method according to the present invention places an S-shaped curve on the non-inverting input of an audio amplifier. The use of an S-shaped curve as a reference voltage greatly reduces the magnitude of the current xe2x80x9cspikexe2x80x9d and the time it takes an audio circuit to recover from the current xe2x80x9cspike.xe2x80x9d Accordingly, the present invention reduces the popping noise in audio circuits while using little additional die space and few additional pins.
The invention provides an audio circuit that includes an amplifier that has an inverting input for receiving an electric input, a non-inverting input for receiving a controlled ground voltage, and an amplifier output which places an output voltage across a load. Each channel in the audio circuit has its own amplifier. The invention also includes a bypass generator that has an output line which is connected to the non-inverting input, and produces on the output line a voltage having a magnitude which is generally a smooth curve, which is preferably an S-shaped curve, when the voltage magnitude is plotted over time. In addition, the audio circuit includes a ground capacitor connected between the output line and a true ground. Furthermore, the audio circuit includes a state machine coupled to the amplifier and the bypass generator. The state machine controls the voltage across the load by placing the audio circuit in three distinct states.
The audio circuit also includes a transistor connected to the bypass generator output line which produces a charge on the ground capacitor. A first comparator and a second comparator are connected to a state machine. The first comparator compares the voltage on the amplifier output to the voltage on the ground capacitor. The second comparator compares the voltage on the amplifier output to the true ground. The amplifier has a state machine-controlled shunt, and an operator controlled variable resistor connected in parallel to provide feedback to the amplifier. Furthermore, in audio amplifiers having loads that are single ended, there is a load capacitor coupled between the load and the amplifier.
In another embodiment, the present invention is configured to provide an audio circuit that includes an integrated circuit (IC) set having an input and an output for a channel, a load connected between the output and ground, and a bypass control connected to the channel block by a bypass control line. A channel block is provided for each channel in the audio circuit. The bypass control provides a generally S-shaped curve to the channel block. A shunt control is coupled between the channel block and the bypass control and controls the voltage across the load, and a ground capacitor is coupled between the bypass control line and ground. The bypass control provides a bypass generator coupled between the shunt control and the ground capacitor as well as a transistor coupled between a voltage source and the ground capacitor (the transistor also has a connection to the state machine, which controls current flow through the transistor). The shunt control provides a state machine, as well as a first comparator and a second comparator. The first comparator has an output connected to the state machine, a first input connected to the ground capacitor, and a second input connected to the channel block. The second comparator has an output connected to the state machine, a first input connected to the ground capacitor, and a second input connected to the channel block, and a third input connected to ground.
The channel block includes an amplifier and a shunt connected between an amplifier output and an inverting input of the amplifier. The channel block also includes an input capacitor connected to the audio circuit input, an input resistor coupled between the input capacitor and the inverting input of the amplifier, and a variable resistor coupled between the inverting input and the amplifier output. Furthermore, the channel block provides a load capacitor coupled between the amplifier output and the load.
In another aspect, the invention is a method of reducing a popping noise in a load connected to an audio circuit. The method includes the steps of supplying a voltage source to the audio circuit, charging a ground capacitor until a voltage across a ground capacitor is at least equal to the voltage across a load capacitor, configuring an amplifier to function as a current sink, sinking a charge on audio circuit capacitors, and then applying a voltage to the load. The voltage applied to the load has a dynamic magnitude that appears as a generally smooth curve, such as an S-shaped curve, when plotted over time.